1. Field of the Invention
This invention relates to yaw measurement devices, that is, devices which measure the deviation of a body from a straight line, and particularly to a system which measures the departure from a straight line of track sections of a track extending through a shaft in a mine.
2. General Description of the Prior Art
The most modern approach to coal mining is longwall mining. In longwall mining, the area to be mined is divided into blocks, perhaps 900 feet across and 5,000 feet long. Two access tunnels are drilled on either side of a block, and a face tunnel is cut between access tunnels at the end of a block. A track for a longwall coal cutting machine, or a longwall shearer, is laid through the face tunnel in a manner that enables the material to be mined and dumped on a moving conveyor which transfers it to one of the access tunnels for removal. The ceiling along the coal face is held up by hydraulic supports that are moved forward after each cut. After the supports are advanced, the ceiling behind the supports falls in. Stresses in the mine roof are critical. If the face becomes bowed, forces in the mine will become excessively unbalanced, and the ceiling sometimes will fall in on the machine. Even if this does not occur, it is possible that the track will lock up because of the forces. For this reason, the face yaw must be measured and controlled and individual track sections moved by adjusted amounts which will enable selected depths of cutting of the tunnel face to maintain an approximately straight line between the access tunnels.
In some longwall mines, visual techniques such as a light beam can be employed to measure deviation of a coal face from a straight line. In others where, for example, the coal seam is not level, and one cannot see from one end of a tunnel to the other, a typical method is to pull a string along the face of the mine and observe the yaw of the mine face from the string. In any event, it appears that none of the prior art methods can be used in all mines, and all of the prior methods require turning off the mining machine while a measurement is being made, and none of these methods appear subject to automation.